Having defined a coordinate system for the patient, we now need to align the pre-operative images to this reference space. This is achieved by defining features in the pre-operative images that can also be identified by our localization device on the surface of the patient. Point-like landmarks are the most common type of feature and these are generally referred to as "fiducials". The fiducials may be purely anatomical points on the skin surface, skin-affixed markers or bone-implanted fiducials. The choice of fiducial depends on the accuracy required by the application.
For early pointer-based guidance we have used anatomical landmarks such as the nasion, the medial and lateral canthi, the external angular processes, the tip of the mastoid process and the occipital protuberance. Experience tells us that these can be located at best to within 3-5 mm, achieving a registration error of similar magnitude. Skin markers have been reported to provide a registration accuracy of 2-4 mm when used with great care. Inaccuracies can occur due to movement of the skin surface either with head repositioning, with application of protective eye covers, or with the force of the pointer used for registration. Care must be taken to protect the upper face after registration, and to mark the center of the marker whilst applying as little force as possible to the skin surface.
The only validated and approved method that provides sub-millimetric accuracy is to use bone-implanted markers . Though this is clearly a rather invasive process, it does provide the most accurate registration for neuronavigation.
Whatever the technology used, patient tracking is relative to some reference frame; the frame is an array of tracked targets that are locked in constant orientation with the patient's head. This can be the reference frame of the device itself, as is the case with mechanical localization. A more common approach is to attach an optical tracker to a Mayfield or similar clamp. To maintain accuracy it is advisable to keep this tracker as close as possible to the surgical field without hampering the procedure. It has also been proposed to attach a tracker either to the patient's palate or the upper teeth . This allows freer movement of the patient's head for interventions where a head clamp is inappropriate. The use of a reference frame is essential,
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